In order to reduce the amount of NOx emissions from gas turbines, a low emissions combustion system is used. A low emissions combustion system, such as an ultra low NOx (ULN) system, comprises a combustor with an annular array of swirlers and a central pilot swirler. Fuel is injected from fuel injectors extending within the swirlers which, along with the central pilot swirler, stabilize the combustion flame. The fuel injectors and swirlers of a typical ULN system are formed as an integral structure mounted to a support housing and located inside a separate combustor basket assembly. A bellmouth structure at the entrance of each swirler increases the uniformity of flow of air entering the swirler by reducing pressure loss. To allow the array of swirlers to be assembled into the combustor basket assembly, an outer section of each swirler bellmouth structure is typically removed. The removed outer sections of the swirlers provide a clearance between the annular array of swirlers and an inner diameter of a combustor cover plate to facilitate assembly of the array of swirlers into the combustor assembly.
Removing an outer section of the swirler bellmouth may create a non-uniformity of airflow at the swirler inlet. The non-uniformity of airflow causes a localized deficiency of axial velocity, which translates into a local fuel rich flow within each swirler, with associated potential emissions and flashback concerns.